1. Formation of iron oxide scale
The Iron oxide scale is a metal oxide attached to the surface formed when a billet is processed in a heated or hot rolled state. This layer of oxide may be a dense ferric oxide or loose ferrous oxide. The iron oxide scale is about 5-15 microns thick, the outermost layer is ferric oxide accounting for 1%, the middle layer is ferrous oxide accounting for 4%, and the innermost layer is ferrous oxide accounting for 95%. The formation of the iron oxide scale is intense at high temperatures, and the formation of the iron oxide scale basically stops when the temperature is lower than 570 °C.
2. Factors affecting the thickness and structure of iron oxide scale
1) Finishing rolling temperature and coiling temperature: Generally, the rolling temperature is controlled at about 850 °C. The higher the temperature, the thicker the oxide scale on the strip, and the content of iron oxide and iron tetroxide, which are difficult to tolerate in the iron oxide scale. higher.
2) Strip specification: the thicker the strip, the thicker the surface iron. The scale of iron oxide on the surface of narrow strip steel is a typical three-layer structure.
3) The position of the iron oxide scale in the steel coil: the head, tail and edge of the strip are in great contact with the air during cooling, and the content of iron sesquioxide and iron tetroxide in the iron oxide scale structure is relatively high.
4) Cooling method: The slower the strip cooling speed, the thicker the iron oxide scale produced, and the insoluble iron oxide and iron tetroxide content in the iron oxide scale are higher.
3. The properties of the iron oxide scale
1) Tightness: The inner layer of the iron oxide scale is a loose and porous fine crystalline structure, mainly composed of ferrous oxide; the middle layer is a dense, non-porous, and cracked magnetic iron oxide (iron tetroxide) with glass-like fractures; The outer layer is iron oxide (iron trioxide) in a crystalline structure.
2) Internal stress: When the internal stress is less than the strength of the iron oxide, the iron oxide scale will crack; when the internal stress is greater than the adhesion between the iron oxide scale and the metal surface, the iron oxide scale will fall off from the metal surface. The rougher the surface of the base iron, the greater the internal stress, and the greater the possibility of the scale breaking and falling off.
3) Adhesion: Adhesion is generally measured by failure stress. The greater the adhesion, the greater the failure stress; the smaller the adhesion, the smaller the failure stress. The failure stress of the outermost layer of ferrous oxide is about 10MPa, the failure stress of the middle layer of ferric oxide is about 40MPa, and the failure stress of the innermost layer of ferrous oxide is about 0.4MPa.
4. Measures to reduce iron oxide scale
1) By reducing the heating temperature, reducing the time in the furnace, and adjusting the atmosphere in the furnace to a partial reducing atmosphere, the formation of furnace pig iron scale is suppressed.
2) By increasing the descaling water pressure, adjusting and optimizing the height and angle of the nozzle, improving the lateral pressure capacity of the vertical roller, and reducing the rough rolling scale.
3) Measures to reduce the scale of the roll raw iron scale: adopt the roll material with good thermal crack resistance, adopt a reasonable grinding system, and promptly and thoroughly remove the residual cracks on the surface of the roll; adopt lubricating rolling to improve the surface quality of the roll and reduce the rolling unit of the stand. Rolling force to prevent the roll surface cracks from expanding due to excessive unit rolling force, resulting in roll raw iron oxide scale; the inlet water volume of the roll cooling water stand is less than the outlet water volume, increasing the cooling water volume of the intermediate stand rolls to ensure rapid cooling of the rolls; Rolling temperature≤1030℃, reduce the roll temperature of the upstream stand of finishing rolling.
4) Water spraying on the side of the finishing stand can reduce the pressing of the iron oxide scale.
5) Improve the quality of mold slag, reduce the involvement of mold slag, and ensure that the descaling effect of the billet can reduce the mold slag iron scale.
6) Optimizing the chemical composition of the billet, on the basis of not affecting the mechanical properties and other properties, minimize the content of C and Si elements, and increase the content of Mn and AL, which is more practical and effective for reducing scale in hot rolling mills.